Long-term potentiation (LTP) and long-term depression (LTD) are well-studied phenomena that may be related to learning and memory. Glutamate is the main excitatory neurotransmitter in the brain. There are at least two types of glutamate receptors, the AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) receptor, and the NMDA (N-methyl-D-aspartate) receptor. The mechanism by which LTP is induced does not involve AMPA receptors, but the synaptic response of potentiation results largely due to AMPA receptor activity, as described in a book by Thompson R. F., titled “Brain: A Neuroscience Primer”, 3rd Edition, published in New York: Worth Publishers, 2000, p. 102-117. The critical event for LTP development occurs when the cell membrane containing the NMDA channels is depolarized sufficiently, Mg2+ leaves the channels and glutamate activation of the NMDA receptors opens the channels, allowing Ca2+ to rush into the neuron. But if LTP involves increased transmitter release from the presynaptic terminals, how could this be caused by activation of NMDA receptors in the postsynaptic membrane? It is only possible if some chemical is released postsynaptically and diffuses back across the synaptic cleft to act on the presynaptic terminals. Some have proposed two candidate substances, which are, nitric oxide and arachadonic acid, as described in a book by Thompson R. F., titled “Brain: A Neuroscience Primer”, 3rd Edition, published in New York: Worth Publishers, 2000, p. 102-117.
Until recently, the role of nitric oxide and cyclic guanidine monophosphate (cGMP), (NO)/cGMP signaling in LTP has remained a matter of debate. Within the cascade, the NO receptor guanylyl cyclase (GC), the cGMP-forming enzyme that is stimulated by NO, plays a key role. Two isoforms of GC (alpha2-GC, alpha1-GC) exist. In a study described by Haghikia A., Mergia E., Friebe A., Eysel U. T., Koesling D., Mittmann T., titled “Long-term potentiation in the visual cortex requires both nitric oxide receptor guanylyl cyclases”, published in Journal of Neuroscience, 2007; 27: 818-823, the contribution of G isoforms to synaptic plasticity was analyzed in knock-out mice lacking either one of the GC isoforms. It was found that LTP induced in the visual cortex is NO dependent in the wild-type mice, absent in either of the GC isoform-deficient mice, and restored with application of a cGMP analog in both strains. The requirement of both NO receptor GCs for LTP, indicates the existence of two distinct NO/cGMP-mediated pathways, which have to work in concert for expression of LTP.
LTP may be present in the cortical regions of the cortico-subcortical neural network involving structures of the dorsal striatum, accumbens, and prefrontal cortex following stimulation of fornix-fimbria bundle, as described in an article by Boeijing a P. H., Mulder A. B., Pennertz C. M., Manshaden I., and Lopes Da Silva F. H., titled “Responses of the nucleus accumbens following fornix/fimbria stimulation in the rat. Identification and long-term potentiation of mono- and polysynaptic pathways”, published in Neuroscience, 1993; 53: 1049-1058. Conversely, in the subcortical region, due to combined activation of other receptors such as the metabotropic receptor, there is a paradoxical long-lasting decrease in the responsiveness of the AMPA receptors to glutamate release, as described in an article by Ito M., titled “Long-term depression”. Annual Review of Neuroscience, 1989; 11: 85-102; as well as in a book by Thompson, R. F., titled “Brain: A Neuroscience Primer”, 3rd Edition, published in New York: Worth Publishers, 2000, p. 102-117.
Neurons and synapses in the mammalian brain exhibit plastic changes, which occur not only during development and under physiological conditions, but also under pathological conditions. Until recently, LTP has only been directly demonstrated in humans in isolated cortical tissue obtained from patients undergoing surgery, where it displays properties identical to those seen in non-human preparations. Inquiry into the functional significance of LTP has been hindered by the absence of a model in the intact human brain. Recently, it was demonstrated that the rapid repetitive presentation of a visual checkerboard (a photic ‘tetanus’) leads to a persistent enhancement of one of the early components of the visual evoked potential in normal humans, as described in an article by Teyler T. J., Hamm J. P., Clapp W. C., Johnson B. W., Corballis M. C., and Kirk I. J., titled “Long-term potentiation of human visual evoked responses.”, published in European Journal of Neuroscience, 2005; 21: 2045-2050. The potentiated response is largest in the hemisphere contralateral to the tetanized visual hemifield and is limited to one component of the visual evoked response (the N1b). This selective potentiation of only the N1b component is not related to overall brain excitability changes, but suggests that the effect is due instead to an LTP process. While LTP is known to exist in the human brain, the ability to elicit LTP from non-surgical patients will provide a human model system allowing the detailed examination of synaptic plasticity in normal subjects and may have future clinical applications in the assessment of cognitive disorders. It had been shown in a work previously described by Clapp W. C., Muthukumaraswamy S. D., Hamm J. P., Teyler T. J., and Kirk I. J., titled “Long-term enhanced desynchronization of the alpha rhythm following tetanic stimulation of human visual cortex”, published in Neuroscience Letters, 2006; 398: 220-223, that a photic tetanus induces LTP-like changes in the visual cortex, as indexed by enhanced event-related desynchronization (ERD) of the alpha rhythm lasting one hour, over occipital electrodes. Because ERD of the alpha rhythm is thought to represent active cortex, these results suggest that the visual tetanus induces long-lasting cortical changes, with stronger neuronal assemblies and increased neuronal output.
There has been a quest for a simple non-invasive model to induce and study LTP and LTD processes in the intact human brain. Little did anyone expect that the head-down bed rest model in conjunction with transcranial Doppler ultrasonography could provide a potential method for the non-invasive induction and recording of LTP and LTD processes. The first attempt to study the effects of head-down bed rest during motor function was described by Njemanze P. C., titled “Cerebral lateralization for motor tasks in simulated microgravity. A transcranial Doppler technique for astronauts.” Journal of Gravitational Physiology, 2002; 9:33-34, and showed that motor function evoked changes in cerebral blood flow velocity that varied in lateralization during head-down bed rest. Further studies revealed that there may be a significant gender-related cerebral asymmetry in response to facial processing during head-down bed rest, as described by Njemanze P. C. titled “Asymmetry in cerebral blood flow velocity with processing of facial images during head-down rest”, published in Aviation Space and Environmental Medicine, 2004; 75:800-805. This study demonstrated unexpected results that showed in men, there was a baseline blood flow lateralization to the right hemisphere, but after 6 hours of head-down bed rest, there was a left lateralization, and after 24 hours of head down bed rest, there remained a left lateralization, and then, a tendency towards right lateralization at one hour in the post head-down bed rest period. In the converse, in women, there was a left lateralization at baseline, then a right lateralization after 6 hours of head-down bed rest. And at 24 hours, there was left lateralization, with a tendency towards no lateralization at one hour in the post head-down bed rest period. However, the latter work as well as other works published in literature, did not clarify if the gender-related response was only restricted to the facial stimuli or was a matter functional asymmetry related to brain processes in general. Particularly, in view of the fact that in another study described by Njemanze P. C., titled “Cerebral lateralization and general intelligence: Gender-differences in a transcranial Doppler study.” Brain and Language, 2005; 92:234-239, it was demonstrated that there was a gender-related cerebral lateralization of general intelligence with right hemisphere asymmetry for general intelligence in men, and left hemisphere asymmetry in women. Further studies were undertaken, which involved color processing in men alone as described by Njemanze P. C., titled “A symmetry of cerebral blood flow velocity response to color processing and hemodynamic changes during −6 degrees 24-hour head-down bed rest in men.” Journal of Gravitational Physiology, 2005; 12(2):33-41. This study demonstrated a right lateralization during color stimulation in men. However, the role of the effects of head-down bed rest remained unclear. Particularly, could head-down bed rest induce long-term potentiation and long-term depression during cognitive brain processes?
The technical and theoretical limitations of the conventional transcranial Doppler ultrasonography did not allow the study of brain processes that could be attributed to LTP or LTD. However, until prior art by Njemanze (US 2004/0158155) disclosed a non-invasive method to determine cerebral blood flow velocity in response to assessment tasks of a human subject, including steps of obtaining a subject's cerebral blood flow velocity in cerebral arteries on both sides of the brain using a microcomputer integrated with a transcranial Doppler ultrasound instrument with two probes placed on the temples and sample volumes focused on cerebral vessels on both sides and calculating laterality index for both arteries. Simultaneously, testing the subject with visual processing tasks presented on the screen of a digital computer while monitoring the mean blood flow velocity during each stage of the task in real-time. Njemanze '155 further discloses processing of the acquired data to determine the spectrum analysis using a microcomputer that is operatively connected to a computer workstation for image retrieval and cross-matching. The method could be used to assess human cognition like memory and learning, which have been associated with long-term potentiation and long-term depression but not demonstrating the induction and recording of LTP and LTD. The process of acquiring data includes first, presenting stimuli (i.e. task) to the subject in intervals of 60 seconds. However, Njemanze '155 fails to provide a model for studying LTP and LTD based on placing the subject on a tilt bed. In addition, Njemanze '155 further fails to teach that the subject is placed head down or head-up at different time intervals as the stimuli are being presented. Although, prior art in view of Falbo (U. S. Pat. No. 6,353,949) disclosed a tilt table in order to position a patient at various angles of incline and decline as desired by the physician, it was not obvious to anyone, and there were no published data in literature that tilting a patient to head-down bed rest could be used to induce LTP and LTD in one hemisphere or the other. While Njemanze (US 2004/0158155) accomplished the development of functional transcranial Doppler spectroscopy (fTCDS) by processing the mean cerebral blood flow velocity data using Fourier time series analysis, the methodology for combined use of fTCDS and head-down bed rest for induction and recording of LTP and LTD was not accomplished. Njemanze '155 teaches that the spectral densities are calculated and plotted from the data of all paradigms/stimuli and the peaks are identified in order to characterize fundamental vascular changes. The cortical (C) or memory peak (M) and sub-cortical peak (S) are identified. The C-peak and S-peak characterize cortical and sub-cortical processes, respectively. However, Njemanze '155 fails to establish a clear method of registering LTP and LTD, namely that the accentuation of the C-peak above baseline values represented processes of cortical long-term potentiation (CLTP), and comparison of recordings at different time intervals, conversely, the attenuation of the C-peak compared to baseline represented cortical long-term depression (CLTD). Similarly, the accentuation of the S-peak above baseline values represented processes of subcortical long-term potentiation (SLTP), and comparison of recordings at different time intervals, conversely, the attenuation of the S-peak compared to baseline represented subcortical long-term depression (SLTD). These findings became evident only after further experimentation described by Njemanze P. C. titled “Asymmetric neuroplasticity of color processing during head down rest: a functional transcranial Doppler spectroscopy study.” Journal of Gravitational Physiology, 2008 15(2):49-59. This work clearly showed that during head-down bed rest, in the right hemisphere but not left, in men, there was simultaneous CLTP and SLTD, wavelength-differencing was absent, but wavelength-encoding was used as cues. There were double luminance effect detectors leading to sensory conflicts. Post head-down bed rest showed reversed wavelength-differencing in both hemispheres, dual luminance effect detectors, CLTP and SLTD. The latter work resolved many of the puzzling issues surrounding experiences of astronauts of light flashes during dark adaptation in Space and proved that fTCDS may be useful in the study of the effects of neuroplasticity of simultaneous color contrast and color constancy in microgravity or simulated microgravity using the head-down bed rest model. The demonstration of the criticality of applying fTCDS to study LTP and LTD processes during head-down bed rest and then head-up bed rest, was a ground-breaking finding, that was totally unexpected but with sound theoretical rationale in hindsight. The rationale is based on the fact that, both vascular and neuronal systems of the brain have identical frequency characteristics, if not, a frequency mismatch would cause gross abnormalities. Therefore, the measurement of the vascular frequency characteristics using Fourier time series analysis would also characterize the frequency responsiveness of the neuronal system, which is otherwise expressed as LTP and LTD processes. Njemanze P. C. in the work titled “Asymmetric neuroplasticity of color processing during head down rest: a functional transcranial Doppler spectroscopy study.” Journal of Gravitational Physiology, 15(2):49-59, 2008, proposed that HDR would elicit a novel sensory or environmental response, and has been associated with NO release from postganglionic nitrergic nerves originating from ipsilateral pterygopalatine ganglion, that innervate the arteries of the circle of Willis as has been described in animal studies by Lee, T. J. Sympathetic modulation of nitrergic neurogenic vasodilation in cerebral arteries. Japanese Journal of Pharmacology, 2002; 88: 26-31 and in another animal study by Okamura, T. K., H. Ayajiki, K. Fujioka, K. Shinozaki, and Toda N. Neurogenic cerebral vasodilation mediated by nitric oxide. Japanese Journal of Pharmacology, 2002: 88: 32-38. This unexpected non-invasive induction of LTP and LTD processes using head-down bed rest model combined with new capabilities of using fTCDS for LTP and LTD detection which was actualized in the present invention was a ground-breaking milestone that were not evident in literature and was not existent in prior art including Njemanze '155, Njemanze '979 and Crutchfield et al. (US 2002/0062078). The improved knowledge resulted from significant new experimentations that advanced the knowledge beyond the possibilities of what was known until March 2007, when the present invention was made. Now, armed with the technology to study non-invasively in an intact human being, the LTP and LTD processes, it became possible to explore other applications.
The use of the present invention to provide a model for the study of LTP and LTD has a wide range of applications for disease diagnosis, therapeutic drug management, research and rehabilitation. One object of the present invention is its application for the study of the effects of drugs of addiction and their remedies. There is accruing evidence to suggest that plasticity-related neuroadaptations within the ventral striatum and related circuitry may depend on glutamate-dopamine interactions. These neuroadaptational changes are concomitants of reinforcement learning, that may underlie drug addiction, as described in an article, in a book by Kelley, A. E., titled “Functional specificity of ventral striatal compartments in appetitive behaviors”. In: Advancing from the Ventral Striatum to the Extended Amygdala. Ed: J. F. McGinty, published in New York: Annals of the New York Academy of Sciences, 1999; 877: 71-90.
Another object of the present invention is to apply the model for study of mechanisms that block LTP processes in sensory chronic pain management and disease conditions of memory deficits. It has been postulated that, activity-dependent changes in synaptic strength may contribute to the formation of memory and the expression of persistent inflammatory pain. Recently, the anterior cingulate cortex (ACC) has been proposed to play an important role for learning, memory and chronic pain. For example, it has been demonstrated that clonidine, a specific alpha2-adrenergic receptor agonist, found to be effective for the treatment of neuropathic pain, may exert analgesic effect by depressing the synaptic plasticity in spinal dorsal horn, via activation of muscarinic receptor-NO-cGMP pathway, as described in an article by Ge Y. X., Xin W. J., Hu N. W., Zhang T., Xu J. T., and Liu X. G., titled “Clonidine depresses LTP of C-fiber evoked field potentials in spinal dorsal horn via NO-cGMP pathway”, published in Brain Research, 2006; 1118: 58-65. The benzodiazepine-diazepam impairs memory and LTP formation in the hippocampus and depresses spinal plasticity related-changes produced by noxious stimulation via activation of the gamma aminobutyric acid GABA(A)-benzodiazepine receptor complex as described in an article by Hu X. D., Ge Y. X., Hu N. W., Zhang H. M., Zhou L. J., Zhang T., Li W. M., Han Y. F., Liu X. G., titled “Diazepam inhibits the induction and maintenance of LTP of C-fiber evoked field potentials in spinal dorsal horn of rats” published in Neuropharmacology, 2006; 50: 238-244.
A further object of the present invention is its application to processes mediated by other GABA-ergic neurons, such as in adaptive eating disorders. The phasic glutamate release could reverse the hyperpolarization of the medium spiny neurons induced by GABA, resulting in a major switch in behavioral patterning for feeding, as described in an article in a book by Kelley, A. E., titled “Functional specificity of ventral striatal compartments in appetitive behaviors”. In: Advancing from the Ventral Striatum to the Extended Amygdala. Ed: J. F. McGinty, published in New York: Annals of the New York Academy of Sciences, 1999; 877: 71-90.
Another object of the present invention is its application for understanding of processes mediated by GABA-ergic neurons, such as those involved in color processing. The phasic glutamate release could reverse the hyperpolarization of the medium spiny neurons induced by GABA, resulting in interference with opponent color processing as described in an article by Njemanze, P. C. titled “Asymmetry of cerebral blood flow velocity response to color processing and hemodynamic changes during −6 degrees 24-hour head-down bed rest in men”, published in Journal of Gravitational Physiology, 2005; 12: 33-41.
Another object of the present invention is its application for understanding of processes mediated by glutamate-dopamine interaction in motor and limbic functions, as described in an article in a book by Healy D. J. and Meador-Woodruff J. H., titled “Glutamatergic modulation of subcortical motor and limbic circuits”. In: Advancing from the Ventral Striatum to the Extended Amygdala. Ed: J. F. McGinty, published in New York: Annals of the New York Academy of Sciences, 1999; 877: 684-687.
Another object of the present invention is the activation of ipsilateral glutamate and NO release using the head-down bed rest (HDR) maneuver. HDR is used for simulation of the cardiovascular effects during Space flights. It has been suggested that HDR exposure results in imbalance between sympathetic vasoconstrictor traffic and vasodilator effects of NO release as described in an article by Kamiya A., Iwase S., Michikami D., Fu Q., Mano T., Kitaichi K., and Takagi K., titled “Increased vasomotor sympathetic nerve activity and decreased plasma nitric oxide release after head-down bed rest in humans: disappearance of correlation between vasoconstrictor and vasodilator” published in Neuroscience Letters, 2000; 281: 21-24, and altered cerebral mean flow velocity (MFV) as described in an article by Njemanze, P. C. titled “Asymmetry of cerebral blood flow velocity response to color processing and hemodynamic changes during −6 degrees 24-hour head-down bed rest in men”, published in Journal of Gravitational Physiology 2005; 12: 33-41. This would suggest that HDR elicited a novel sensory or environmental response and has been associated with NO release from postganglionic nitrergic nerves originating from ipsilateral pterygopalatine ganglion, as described in articles by Lee, T. J., titled “Sympathetic modulation of nitrergic neurogenic vasodilation in cerebral arteries”, published in Japanese Journal of Pharmacology, 2002; 88: 26-31; and by Okamura, T. K., H. Ayajiki, K. Fujioka, K. Shinozaki, and Toda N., titled “Neurogenic cerebral vasodilation mediated by nitric oxide”, published in Japanese Journal of Pharmacology 2002:88:32-38. It is plausible that, the NO released diffuses back across the synaptic cleft to act on presynaptic terminals, to cause increased release of glutamate, as described in a book by Thompson, R. F., titled “Brain: A Neuroscience Primer”, 3rd Edition, published in New York: Worth Publishers, 2000, p. 102-117. The changes in MFV induced by HDR suggest a left lateralization but relative right hypoperfusion. Therefore, the expected phasic glutamate release that overcomes GABA-mediated cortical inhibition leading to sustained ipsilateral LTP processes would occur in the cortical regions of the right hemisphere.
Another object of the present invention is to assess the state of brain autoregulation during unconsciousness. Autoregulation refers to the capability of the cerebrovascular system to maintain normal cerebral perfusion despite fluctuation in arterial blood pressure. Failure of brain autoregulation results in unconsciousness with fall in arterial blood pressure. HDR induced during normal function of brain autoregulation results in cerebral hypoperfusion as described in an article by Njemanze, P. C. titled “Asymmetry of cerebral blood flow velocity response to color processing and hemodynamic changes during −6 degrees 24-hour head-down bed rest in men”, published in Journal of Gravitational Physiology, 2005; 12: 33-41. However, HDR is a well known maneuver for inducing brain reperfusion during unconsciousness, in a failed state of autoregulation. It is the object of the present invention to determine at which point there is restoration of normal brain perfusion that sustains cortical LTP and subcortical LTD.
A further object of the present invention is to provide a method that analyzes MFV data obtained in real-time by Fourier algorithm. The obtained spectral density estimates are plotted to determine the peaks for cortical and subcortical responses using an algorithm called functional transcranial Doppler spectroscopy (fTCDS) as described in an article by Njemanze P. C., titled “Cerebral lateralization for facial processing: Gender-related cognitive styles determined using Fourier analysis of mean cerebral blood flow velocity in the middle cerebral arteries”, published in Laterality, 2007; 12: 31-49.